Tuesday, 10 January 2017

How much difference can we make if we change our diets?

Our food choices make a big difference to carbon emissions. In particular the amount of meat and animal products we eat is important. How much do we need to change to make a difference? I have been looking at data from the National Diet and Nutrition Survey [1] to see what we eat and where the carbon impact is. I have also derived an alternative diet which is not meat free but still saves a third of a tonne per year in greenhouse gas emissions.

Beef, lamb and dairy products are the worst for GHG emissions because ruminants belch methane, a very powerful greenhouse gas. However, there are emissions from all livestock, mainly to do with manure management and feed production. So all meat is high in emissions but some types are much worse than others.

We get more protein from bread and pasta than from beef.
In our current diet I found some surprises. Did you know that we get more protein from bread and pasta than we do from red meat? The total from ruminants, including dairy as well as red meat, is less than 25%. We eat a lot of chicken. (Obviously this is only an average. People are very varied in their dietary choices.)

We eat 20-30% more protein than we need.
We also eat a lot more protein than we need. Men eat around 85g daily and only need 55g while women eat 64g and need only 45g [2]. In my alternative diet the amount of protein is a compromise about half way between what we consume now and what we actually need.

In our current diet, red meat is responsible for the most GHG emissions, even though it provides us with a small proportion of our protein.
This chart shows where the carbon emissions are in an average (age 18-60) man's diet and the suggested alternative. The area of the rectangles illustrates the overall greenhouse gas emissions. The largest single contribution in the current man's diet is red meat, even though this is only 7.6g protein (30g meat)/day.

GHG emissions for an average UK man's diet compared to an alternative with less meat. The area of the rectangles indicates the overall emissions for each component of the diet. Diet and protein from [1], GHG emissions mostly from [3].

The alternative diet has more eggs, beans, nuts and seeds.
In the alternative diet, protein from meat sources is reduced by about 2/3. In terms of actual meat consumption this is from 123g/day to 44g. So you might eat meat every two or three days instead of every day. The alternative diet also has less dairy products, but more protein from eggs, vegetables (e.g. beans, peas and other legumes), nuts and seeds.

The alternative diet is similar in calories.
Meat, especially chicken, is very low in calories per unit protein, whereas alternative proteins tend to have more calories. If you have ever been on a diet to lose weight you probably know this already. For example, red meat is typically 10 kCal/g protein whereas eggs are about 11.5 and vegetables sources typically 15. Nuts and seeds are worst, with 30 kCal/g protein! Peanut butter is an excellent source of protein from the environment point of view, but not if you are trying to lose weight.

Fortunately, since we need less protein than we are eating now, the higher calories are not so critical. The total number of calories in the alternative diet is hardly changed from the current one.

Women save a third of tonne GHG emissions/year; men nearly half a tonne.
Women eat less than men so the GHG emissions savings are a bit less too. The table below shows the total figures for GHG emissions and savings for both men and women.

GHG emissions
GHG emissions savings
Actual average man
Suggested man
Actual average woman
Suggested woman

Too much meat is boring
I know a lot of vegetarians and vegans but I classify myself as a 'low meat' consumer. I generally eat meat only 2-3 times per week and frankly I find a meat oriented diet rather boring.

My favourite lentil cake recipes: (1) tasty, chewy and crisp (2) tasty and soft

We eat a lot of lentils because they are conveniently quick to cook and versatile. We use them in curry, soup and hummus amongst other ways. I recently discovered some excellent recipes for fried lentil cakes. My favourite is an adaption of 'South Indian Lentil cakes with Raita' (www.bonappetit.com). To make them you soak uncooked lentils and rice together to soften the grains, then process them to a paste and add spices and other stuff. The result is tasty, chewy and crispy. But we also like 'Lentil cakes with creamy horseradish sauce' (www.peta.org). These are made from cooked rice and lentils mixed with other stuff and coated in breadcrumbs before frying. They are differently tasty and soft inside.

There are lots of sources for meat free recipes. For example Meat Free Mondays and BBC Good Food. Italian dishes such as pasta with sauce and frittata are conveniently low in meat or adaptable. Pizza and risotto are infinitely variable. Indian cooking has a huge range of vegetarian recipes and we adapt ideas from Thai and Middle Eastern traditions too. Have fun!

[1] National Diet and Nutrition Survey Years 5 and 6 (www.gov.uk) Sep. 2016
[2] Government Dietary Recommendations (Public Health England) Aug. 2016
[3] Climate and Environmental Impacts from Meat Eaters Guide: Report (Environmental Working Group) 2011


  1. Is rice not also a high carbon food?

    1. That is an interesting point. The Environmental Working Group rates it as low in emissions - about the same as potatoes. I think the main issue with rice is methane emissions from paddy fields. Methane is a very powerful GHG but does not last long. If you use the 100 year global warming potential this is is not so bad but over shorter time scales rise is much worse than potatoes.

      There are ways to reduce methane emissions from paddy. For example alternate wetting and drying (AWD) can cut emissions by about 50% and also uses less water. See https://ccafs.cgiar.org/publications/alternate-wetting-and-drying-irrigated-rice#.WHYX1rbyjSd

    2. Also, I just tried making the soft lentil cakes recipewith mashed potato instead of cooked rice. This was a bit smoother but still very nice.

  2. We can always count on you to assimilated a difficult-to-research subject and present it in a clear way. Thanks for that!


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